Today's petroleum refinery processes include many complex operations, one or more of which may require controlled regulation of process streams as a function of measured system parameters. For example, in a complex hydrocarbon flash distillation process, an intermediate liquid side steam drawn from liquid condensate within a complex distillation column at, say a withdraw zone, may be used for a multiplicity of purposes, inter alia, (i) to produce subsequent jet fuel product in a downsteam liquid-vapor stripping process, (ii) to control flow, and hence, efficiency in side stream gas recovery columns, reboilers, and the like, and (iii) to control intermediate recycle circulation of the distillation column itself. Remaining excess liquid condensate within the column, called overflash, is free to cascade down to lower fractionation trays of the column located below the withdrawal zone. In providing items (i)-(iii), above, the flow rate of the side stream exiting from the column has been found to be a valuable parameter for control purposes. However, experience also has shown that if the established withdraw rate becomes unbalanced vis-a-vis the flow rate of the overflash at the withdrawal zone, there can be upsets in the associated processes linked to the withdrawal line as well as a reduction in fractionation efficiency within the distillation column itself. See Shinskey, F. G., "The Value of Process Control" Oil and Gas Journal, Feb. 18, 1974 and articles cited therein.
While instrumentation for measuring process parameters in the external piping is feasible, response time for such equipment is often inadequate to properly control the complex processing system, at least in the situation where deviation from selected setpoint limits occurs at a rather rapid rate.